This invention relates generally to a dark current correction circuit in a two-tube type color television camera. More particularly, the invention relates to a dark current correction circuit which, in a color television camera of two-tube type having a camera tube for chrominance signals and a camera tube for luminance signals, eliminates the effect of dark current variations due to temperature variations due to temperature variations of the two camera tubes.
In general, camera tubes of the vidicon class such as vidicons, Plumbicons, and Newbicons, are accompanied by the problems of much dark current and large variation of dark current occurring particularly when there are temperature variations.
In a color television camera, if the setup (black level as a reference) of a chrominance signal fluctuates, the white balance will be disturbed, and therefore it is necessary to prevent such fluctuation of the setup. When there is a temperature variation, the dark current of the camera tube also varies, and for this reason, the setup fluctuates. Accordingly, for the purpose of substantially preventing variation of the setup even when the dark current of the camera tube varies, a dark current detection circuit and dark current compensation circuit have heretofore been used.
In such a circuit organization, a masking plate is placed at one part of the image pickup surface of the camera tube, and the dark current variation is compensated or corrected for by subjecting the signal of that part corresponding to this plate (that is, the dark current part) to sampling by the dark current detection circuit and subjecting this to subtraction with the image pickup output signal by means of the dark current compensation circuit.
In the case of a monochrome (black-and-white) television camera, the problem of the above described adverse effect due to fluctuation of the setup does not arise, but in a color television camera, the above described countermeasure becomes necessary. In the case of a two-tube type color camera, it is necessary to provide the above mentioned dark current detection circuit and dark current compensation circuit in the signal system of the camera tube for chrominance signals. For this reason, there arises the necessity of setting the black level also with respect to the signals of the camera tube for luminance signals.
Accordingly, the solution would seem to be the provision of a dark current detection circuit, a dark current compensation circuit, etc., similar to those of the chrominance signal system, also for the signal system of the camera tube for luminance signals. This measure, however, would give rise to a complication of the circuit organization and an increase in the cost of production of the camera.
On one hand, when there is a temperature rise in the image pickup surface of a camera tube, the dark current increases over the entire image pickup surface, and, at the same time the static shading (that is, a condition wherein the dark current is not uniform over the entire image pickup surface but is large particularly in the end parts) increases. For this reason, the dark current detection circuit detects also the resulting increase in this shading, whereby an accurate dark current correction cannot be carried out. Accordingly, as one countermeasure, the target voltage of the camera tube is lowered at the time of a temperature rise thereby to suppress the increase in the dark current, and, at the same time, to suppress the increase in the shading.
In a two-tube color camera, however, if the above mentioned target voltage control is carried out in only the camera tube for chrominance signals, there will arise the problem of difference between the sensitivities of the two camera tubes for chrominance signals and for luminance signals.